1. Field of the Invention
This invention relates generally to a fuel cell and, more particularly, to a fuel cell that includes a decomposition catalyst that decomposes hydrogen peroxide to reduce membrane degradation.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer-electrolyte proton-conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs require certain conditions for effective operation, including proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For the automotive fuel cell stack mentioned above, the stack may include about two hundred fuel cells. The fuel cell stack receives a cathode reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack.
The fuel cell stack includes a series of flow field or bipolar plates positioned between the several MEAs in the stack. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode gas to flow to the anode side of the MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode gas to flow to the cathode side of the MEA. The bipolar plates also include flow channels through which a cooling fluid flows.
The bipolar plates are typically made of a conductive material, such as stainless steel, titanium, aluminum, polymeric carbon composites, etc., so that they conduct the electricity generated by the fuel cells from one cell to the next cell and out of the stack. Metal bipolar plates typically produce a natural oxide on their outer surface that makes them resistant to corrosion. However, the oxide layer is not conductive, and thus increases the internal resistance of the fuel cell, reducing its electrical performance. Also, the oxide layer makes the plate more hydrophobic.
Oxygen permeates through the membrane from the cathode side to the anode side of the fuel cells in the stack, sometimes referred to as oxygen cross-over. The reduction of oxygen on the anode and anode sides of the fuel cell generates hydrogen peroxide. The hydrogen peroxide reacts vigorously with ferric and/or ferrous ions that are generated by corrosion of the bipolar plates. The reaction of hydrogen peroxide and ferrous ions produces hydroxyl free radicals, which degrade the membrane as a result of Fenton's reaction shown below.H2O2+Fe2+—>Fe3++HO−+HO*  (1)RH+OH*—>R*+H2O  (2)R*+Fe3+—>R++Fe2+  (3)
The degradation of the membrane produces hydrogen fluoride in the fuel cell. The hydrogen fluoride corrodes the bipolar plates, which generates more ferric and ferrous ions, increasing the production of the hydroxyl free radicals, and thus further increasing the degradation of the membrane. This process thus becomes auto-catalytic and a significant degradation of both the membrane and the bipolar plate occurs as a result of the reaction.